This invention relates generally to gas turbine engines, and in particular, to a process for cooling a flow path surface region on a turbine airfoil.
In gas turbine engines, for example, aircraft engines, air is drawn into the front of the engine, compressed by a shaft-mounted rotary-type compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on a shaft. The flow of gas turns the turbine, which turns the shaft and drives the compressor and fan. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward.
During operation of gas turbine engines, the temperatures of combustion gases may exceed 3,000xc2x0 F., considerably higher than the melting temperatures of the metal parts of the engine aft of the compressor, which are in contact with these hot gases. Operation of these engines at gas temperatures that are above the metal part melting temperatures is a well established art, and depends in part on supplying a cooling fluid to the outer surfaces of the metal parts through various methods. Metal parts of these engines that are particularly subject to high temperatures, and thus require particular attention with respect to cooling, are, for example, combustor liners and the metal parts located aft of the combustor including high pressure turbine airfoils, such as turbine blades and turbine vanes.
The hotter the turbine inlet gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the turbine inlet gas temperature. However, the maximum temperature of the turbine inlet gases is normally limited by the materials used to fabricate the components downstream of the combustors such as the vanes and the blades of the turbine. In current engines, the turbine vanes and blades are made of nickel-based superalloys, and can operate at temperatures of up to 2100xc2x0-2200xc2x0 F. with appropriate well-known cooling techniques.
The metal temperatures can be maintained below their melting levels with current cooling techniques by using a combination of improved active cooling designs and thermal barrier coatings (TBCs). For example, with regard to the metal blades and vanes employed in aircraft engines, some cooling is achieved through convection by providing passages for flow of cooling air from the compressor internally within the blades so that heat may be removed from the metal structure of the blade by the cooling air. Such blades have intricate serpentine passageways within the structural metal forming the cooling circuits of the blade.
Small internal orifices have also been devised to direct this circulating cooling air directly against certain inner surfaces of the airfoil to obtain cooling of the inner surface by impingement of the cooling air against the surface, a process known as impingement cooling. In addition, an array of small holes extending from a hollow core through the blade shell can provide for bleeding cooling air through the blade shell to the outer surface where a film of such air can protect the blade from direct contact with the hot gases passing through the engines, a process known as film cooling.
In another approach, a TBC is applied to the turbine blade component, which forms an interface between the metallic component and the hot gases of combustion. The TBC includes a ceramic coating that is applied to the external surface of metal parts to impede the transfer of heat from hot combustion gases to the metal parts, thus insulating the component from the hot combustion gas. This permits the combustion gas to be hotter than would otherwise be possible with the particular material and fabrication process of the component.
TBCs include well-known ceramic materials, such as, for example, yttrium-stabilized zirconia (YSZ). Ceramic TBCs usually do not adhere well directly to the superalloys used as substrate materials. Therefore, an additional metallic layer called a bond coat is placed between the substrate and the TBC. The bond coat may be made of an overlay alloy, such as a MCrAIX, or other composition more resistant to environmental damage than the substrate, or alternatively, the bond coat may be a diffusion nickel aluminide or platinum aluminide. The surface of the bond coat oxidizes to form a thin, protective aluminum oxide scale that provides improved adherence to the ceramic top coatings. The bond coat and overlying TBC are frequently referred to as a thermal barrier coating system.
Improved environmental resistance to destructive oxidation and hot corrosion is desirable. Additionally, the alloying elements of the bond coat interdiffuse with the substrate alloy at elevated temperatures of operation, changing the composition of the protective outer layer. Over time, as the airfoils are refurbished, walls of the airfoils are consumed, which reduces load carrying capability and limits blade life. Also, this interdiffusion can also reduce environmental resistance of the coating, causing loss of material, as layers of material are lost due to corrosive and oxidative effects. This interdiffusion and its adverse effects can be reduced by controlling the temperature of the component in the region of the bond coat/substrate interface.
In previous and existing designs, the bond coat temperature limit is critical to the TBC""s life and has had an upper limit of about 2100xc2x0 F. Once the bond coat exceeds this temperature, the coating system will quickly deteriorate, due to high temperature mechanical deformation and accelerated oxidation, as well as from accelerated interdiffusion of bond coat elements with those from the substrate alloy and subsequent degradation due to loss of its superior environmental resistance. The coating system ultimately can separate from the substrate exposing the underlying superalloy component to further deterioration from the hot gases.
Even with the use of advanced cooling designs and thermal barrier coating systems, it is also desirable to decrease the requirement for cooling fluid, because reducing the demand for cooling fluid also contributes to improving overall engine operating efficiency. One way to achieve such a reduction is to improve the current cooling techniques for the metal parts immediately adjacent to their outer surfaces.
A process of cooling these metal parts has been set forth in a co-pending application identified as Attorney Docket No. 13DV-13513 entitled xe2x80x9cDirectly Cooled Thermal Barrier Coating Systemxe2x80x9d, in which micro channels were created within or adjacent to the bond coat layer. Alternatively, when formed adjacent to or within a porous TBC, the micro channels additionally provide transpiration cooling through the porous TBC. The micro channels communicate directly with at least one cooling circuit contained within the blade or vane from which they receive cooling fluid, thereby providing direct and efficient cooling for the bond coat layer. The micro channels may be parallel to one another or they may intersect to form a cooling mesh. In this manner, the component includes an actively cooled flow path surface region that can improve the cooling of the substrate without increasing the demand for cooling fluid, and the engine can run at a higher firing temperature without the need for additional cooling fluid, thereby achieving a better, more efficient engine performance.
These micro channels are formed by masking the substrate surface with a masking material in a preselected pattern. The masking material permits the formation of a pattern, upon application of material, on the surface overlying at least one cooling fluid supply circuit contained within the component. The masking material is subsequently removed, leaving hollow micro channels to actively cool the flow path surface region. Depending on the desired location of the micro channels, the masking material may be placed directly on the superalloy substrate, then covered with the bond coat. Alternatively, the bond coat may be applied to the superalloy component followed by placement of the masking material on the bond coat, which now serves as a substrate. Alternatively, TBC system may be employed wherein the masking material is placed upon a deposited layer of TBC, which then serves as the substrate for formation of micro channels.
Utilizing a masking material that must later be removed adds additional manufacturing time and expense. Thus, there is an ongoing need for improved methods for economically creating micro channels used to improve the environmental resistance and long-term stability of the thermal barrier coating system, so that higher engine efficiencies can be obtained. The present invention fulfills this need, and further provides related advantages.
The present invention provides a process of creating active cooling micro channels within or adjacent to a bond coat layer without the use of masking materials. When placed adjacent to or within a porous TBC, the micro channels allow for cooling of the bond coat while additionally provide transpiration cooling through the porous TBC. The micro channels are positioned to communicate directly with at least one cooling circuit internal to the turbine engine component, which cooling circuit is the source of cooling fluid to the micro channels, thereby providing direct and efficient cooling for the bond coat layer. Because these micro channels provide an actively cooled flow path surface region for the substrate that can reduce the cooling requirement for the substrate or improve the cooling capability of the substrate using existing cooling fluid requirements, the engine can run at a higher firing temperature without the need for additional cooling air, achieving a better, more efficient engine performance.
In one embodiment, the present invention utilizes the unique line of sight feature of electron beam physical vapor deposition (EB-PVD) to deposit a material onto a surface containing structured micro grooves that have been fabricated into the surface. The shadowing effect of the subsequent material deposits on the peaks of the micro grooves and bridging over the valleys creates micro channels as the material deposits over the structured micro grooves.
When fabricated adjacent to and in fluid communication with, or within a porous TBC, the micro channels provide both active and transpiration cooling through the porous TBC. The micro channels are placed to communicate directly with at least one cooling circuit contained within the jet engine turbine component, from which they receive cooling fluid, thereby providing direct and efficient cooling for the TBC system. The result is a substrate having an actively cooled flow path surface region that can reduce the cooling requirement for the component.
The present invention further comprises the cooled flow path surface region of the jet engine turbine component formed by the foregoing processes and the turbine component with the patterned micro channels formed in the surface region by the foregoing methods for cooling the component. The location of the micro channels may be at any preselected position at the flow path surface or in the vicinity of the flow path surface region, for example, within the airfoil substrate at the substrate/bond coat interface, within the bond coat at the bond coat/TBC interface, or entirely within a TBC layer.
An advantage of the present invention is the micro channel that provides active cooling to the flow path surface region of the coated gas turbine component is fabricated in a cost-effective manner.
In one embodiment, the active convection cooling through the micro channels occurs within or adjacent to the bond coat layer, providing direct and efficient cooling for the bond coat layer. The substrate, upon which is deposited the bond coat layer, will also benefit from the cooling provided by the micro channels.
Another advantage of the present invention is that the actively cooled bond coat layer will allow engine components to run at higher operating temperatures to achieve a better engine performance. By removing heat from this region, the integrity of the bond coat can be maintained at higher engine operating temperatures, and diffusion of the bond coat elements with elements from the substrate will be retarded.
Still another advantage of the present invention is that cooling air flowing through the porous TBC will assist in lowering the TBC temperature by removing heat, thereby improving the TBC""s thermal insulation efficiency.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying figures which illustrate, by way of example, the principles of the invention.